On Wed, Feb 28, 2018 at 9:32 AM, Marek Polacek <pola...@redhat.com> wrote: > On Tue, Feb 27, 2018 at 04:16:31PM -0500, Jason Merrill wrote: >> On 02/27/2018 02:13 PM, Marek Polacek wrote: >> > My recent change introducing cxx_constant_init caused this code >> > >> > template <class> class A { >> > static const long b = 0; >> > static const unsigned c = (b); >> > }; >> > >> > to be rejected. The reason is that force_paren_expr turns "b" into >> > "*(const >> > long int &) &b", where the former is not value-dependent but the latter is >> > value-dependent. So when we get to maybe_constant_init_1: >> > 5147 if (!is_nondependent_static_init_expression (t)) >> > 5148 /* Don't try to evaluate it. */; >> > it's not evaluated and we get the non-constant initialization error. >> > (Before we'd always evaluated the expression.) >> > >> > Bootstrapped/regtested on x86_64-linux, ok for trunk? >> > >> > 2018-02-27 Marek Polacek <pola...@redhat.com> >> > >> > PR c++/84582 >> > * semantics.c (force_paren_expr): Avoid creating a static cast >> > when processing a template. >> > >> > * g++.dg/cpp1z/static1.C: New test. >> > * g++.dg/template/static37.C: New test. >> > >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c >> > index 35569d0cb0d..b48de2df4e2 100644 >> > --- gcc/cp/semantics.c >> > +++ gcc/cp/semantics.c >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr) >> > expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr); >> > else if (VAR_P (expr) && DECL_HARD_REGISTER (expr)) >> > /* We can't bind a hard register variable to a reference. */; >> > - else >> > + else if (!processing_template_decl) >> >> Hmm, this means that we forget about the parentheses in a template. I'm >> surprised that this didn't break anything in the testsuite. In particular, >> auto-fn15.C. I've attached an addition to auto-fn15.C to catch this issue. > > Thanks, you're right. I'll use it. > >> Can we use PAREN_EXPR instead of the static_cast in a template? > > I don't think so, it would fix the issue you pointed out in auto-fn15.C but > it wouldn't fix the original test. The problem with using PAREN_EXPR in a > template is that instantiate_non_dependent_expr will turn in into the > static cast anyway; tsubst_copy_and_build has > case PAREN_EXPR: > RETURN (finish_parenthesized_expr (RECUR (TREE_OPERAND (t, 0)))); > so it calls force_paren_expr and this time we're not in a template. And > then when calling cxx_constant_init we have the same issue.
Then maybe we need something like fold_non_dependent_expr, which checks for dependency before substitution and then immediately evaluates the result. Jason